Process for the treatment of waste water

ABSTRACT

A process for treating waste water streams containing at least hydrocarbons and salts to yield a clean water product, a concentrated brine product and a hydrocarbon-rich product, which process includes the steps of: (a) feeding the waste water feed into a first distillation column at a stage in the range of from 0.05 to 0.15 from the top, wherein n represents the total number of theoretical stages of the first distillation column and has a value in the range of from 20 to 40; (b) drawing off a vapor stream at a stage in the range of from 0.55 to 0.75 from the top and feeding this vapor stream into the bottom of a second distillation column having m theoretical stages, with m having a value in the range of from 3 to 10; (c) drawing off the clean water product as the top fraction from the second distillation column and drawing off a bottom stream from the second distillation column, which is fed back into the first distillation column below the draw off of the vapor stream in step (b) at a stage in the range of from 0.60 to 0.85 from the top; (d) drawing off the concentrated brine product as the bottom fraction of the first distillation column; and (e) drawing off the hydrocarbon stream as the top fraction of the first distillation column.

FIELD OF THE INVENTION

The present invention relates to a process for the treatment of wastewater, in particular waste water from industrial processes containing atleast hydrocarbons and salts.

BACKGROUND OF THE INVENTION

The treatment or purification of waste water originating from industrialprocesses and containing at least hydrocarbons and salts (organic and/orinorganic) normally is a relatively expensive procedure. Environmentallegislation nowadays puts stringent demands on the purification or wastewater streams from industrial processes, particularly when the purifiedwaste water is to be released into the environment. Accordingly, thechoice of a purification method for industrial waste water is bound bypractical, environmental and economic considerations.

One industrial process, wherein a relatively large amount of waste wateris produced is the styrene monomer/propylene oxide (SM/PO) productionprocess. In general such SM/PO process involves the steps of: (i)reacting ethylbenzene with oxygen or air to form ethylbenzenehydroperoxide, (ii) reacting the ethylbenzene hydroperoxide thusobtained with propene in the presence of an epoxidation catalyst toyield propylene oxide and 1-phenyl ethanol, and (iii) converting the1-phenyl ethanol into styrene by dehydration using a suitabledehydration catalyst. In the last step water is produced. In addition tothis reaction water organic by-products such as aliphatic and aromatichydrocarbons, aldehydes, ketones, alcohols, phenols and organic acidsare produced. The by-products are separated from the main products withthe aid of clean water and the organic acids are neutralized using abasic aqueous solution, such as an aqueous sodium (bi)carbonate and/orsodium hydroxide solution. Furthermore, additional water is introducedwith the air in the step (i) and as steam in step (iii) of the aboveprocess.

The waste water from an SM/PO production plant typically contains atotal of from 1.0 to 3.5 wt % of non-salt organic compounds and from 3.0to 6.0 wt % of organic salts. It may further contain up to 2.0 wt % ofsodium carbonate and sodium bicarbonate and/or traces of sodiumhydroxide, depending on the basic solution used in the neutralization oforganic acids.

The input of clean water to an SM/PO plant can be up to tens ofthousands kg per hour, while the output of waste water is normally about50% higher than the imput of clean water. The waste water cannot bedischarged without additional purification treatment. As has alreadybeen indicated above, however, the choice of a suitable purificationtreatment is limited due to all sorts of practical, environmental andeconomic considerations.

In GB-A-2,252,052 further information is given about prior art methodsfor treating waste water and about typical compositions of SM/PO wastewater streams. The purification process disclosed in GB-A-2,252,052involves freeze-concentration combined with salts-removal, whereby thewaste water is separated into an at least two-fold concentrated wasteproduct, salt crystals and a substantially pure water product.

The method disclosed in GB-A-2,252,052, however, still leaves room forimprovement. Particularly the economics of a freeze concentrationprocess are still not satisfactorily. Capital investment required for afreeze concentration process at present account for at least 10% of thetotal capital investment necessary for an SM/PO plant. It would thus bebeneficial if a cheaper, but at least equally good—in terms ofpurification—alternative waste water purification treatment could bedeveloped. Thus, it is an important object of the present invention todevelop such alternative. A further object is to develop a waste watertreatment which produces a clean water stream, which meets allrequirements for discharge into the environment.

It has been found in accordance with the present invention that byapplying a specific distillative treatment a very effective purificationof the waste water is accomplished at reduced cost.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a process for treating ofwaste water streams containing at least hydrocarbons and salts to yielda clean water product, a concentrated brine product and ahydrocarbon-rich product, which process comprises the steps of:

(a) feeding the waste water feed into a first distillation column at astage in the range of from 0.05*n to 0.15*n from the top, wherein nrepresents the total number of theoretical stages of the firstdistillation column and has a value in the range of from 20 to 40;

(b) drawing off a vapour stream at a stage in the range of from 0.55*nto 0.75*n from the top and feeding this vapour stream into the bottom ofa second distillation column having m theoretical stages, with m havinga value in the range of from 3 to 10;

(c) drawing off the clean water product as the top fraction from thesecond distillation column and drawing off a bottom stream from thesecond distillation column, which is fed back into the firstdistillation column below the draw off of the vapour stream in step (b)at a stage in the range of from 0.60*n to 0.85*n from the top;

(d) drawing off the concentrated brine product as the bottom fraction ofthe first distillation column; and

(e) drawing off the hydrocarbon stream as the top fraction of the firstdistillation column.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an illustrative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The above process has many advantages. First of all, it is highly costeffective. Capital investment required for the above purificationtreatment is at least 10% lower, but may be as high as 50% lower thanfor the freeze concentration process disclosed in GB-A-2,252,052.Furthermore, from the viewpoint of purification it also performsexcellent. The clean water obtained meets all specifications in terms ofChemical Oxygen Demand (COD) and harmful components for direct dischargeinto the environment. As is well known in the art, COD is a measure forthe quantity of contaminating components in a process stream, moreparticularly for the amount of oxygen necessary to oxidise (and hencedegrade) the contaminating components present in a process stream. CODis typically expressed in milligrams of oxygen per litre of processstream. As a rule a purified waste water stream should have a COD of atmost 100 mg/l before it can be discharged into the environment. Harmfulcomponents mainly are phenolic components like phenol and phenolates.Typically, such components must be present in a purified process streamin an amount of less than 50 ppm before it is allowed to discharge thisstream into the environment. The hydrocarbon-rich product obtained isvery suitable as fuel or can be re-used in the process from which thewaste water stream originates. The concentrated brine product can beincinerated in conventional incineration installations.

The waste water stream to be treated by the process according to thepresent invention should at least contain hydrocarbons and salts. Anyindustrial process yielding such type of waste water could benefit fromthe purification process according to the present invention. It has,however, been found particular advantageous to treat waste water streamsoriginating from a styrene monomer/propylene oxide production plant.Such waste water streams typically comprise hydrocarbons, aldehydes,ketones, alcohols, phenols and salts.

The first distillation column has from 20 to 40 theoretical stages.Preferably, the number of theoretical stages (n) of the firstdistillation column is in the range of from 25 to 35. The number ofactual trays to be used in the distillation column can be easilydetermined on the basis of the efficiency of the trays used and thenumber of theoretical stages. In step (a) of the process, the wastewater stream enters the first distillation column at a stage which is0.05*n to 0.15*n from the top. Preferably the waste water enters thecolumn right below the top at the second, third, fourth or fifth stagefrom the top.

In step (b), subsequently, a vapour stream is drawn off at a stage inthe range of from 0.55*n to 0.75*n, preferably from 0.60*n to 0.70*n,from the top. The vapour stream is fed into the bottom of a seconddistillation column having m theoretical stages, with m having a valuein the range of from 3 to 10. The preferred number of theoretical stagesof this second distillation column is 3 to 7.

In step (c) the clean water product is withdrawn from the seconddistillation column as the top fraction. The bottom stream from thissecond distillation column is fed back into the first distillationcolumn below the draw off of the vapour stream in step (b) at a stage inthe range of from 0.60*n to 0.85*n, preferably from 0.65*n to 0.75*n,from the top. Accordingly, it is preferred that steps (b) and (c) beoperated such that the vapour stream: is drawn off in step (b) at astage in the range of from 0.60*n to 0.70*n and that the bottom streamfrom the second distillation is fed back into the first distillationcolumn in step (c) at a stage in the range of from 0.65*n to 0.75*n fromthe top.

The distillation treatments in the first and/or the second distillationmay be carried out under atmospheric pressure or under reduced pressure.It is, however, preferred to operate both distillation columns underatmospheric conditions.

Each distillation column suitably has at least one reboil duty and acondensing duty to increase the separation efficiency. As regards thefirst distillation column, the temperature regime must be such that thevapour stream, mainly consisting of water, can be withdrawn at a stagein the range of from 0.55*n to 0.75*n from the top. This implies thatthe temperature in the first distillation column should be at least 100°C. Since in the second distillation column the clean water stream isrecovered as the top fraction, the temperature in this distillationcolumn should also be at least 100° C. In practice, it has been foundthat both columns are suitably operated under atmospheric conditions ata temperature of from 100° C. to 110° C. By operating under theseconditions the reboil duty in the first distillation column can beprovided by low level heat, which could be low pressure steam or wasteheat from the industrial process from which the waste water streamoriginates. The waste water entering the first distillation columnsuitably has a temperature in the range of from 50 to 110° C., moresuitably 65 to 90° C.

FIG. 1 shows an illustrative embodiment of the present invention.

Waste water stream 1 enters the first distillation column 2. Topfraction 3 is passed through heat exchanger 4 and the resulting at leastpartly condensed stream 5 is separated in phase separator 6 intohydrocarbon-rich stream 7 and a stream 8 containing a major proportionof water, the latter stream being fed back into the first distillationcolumn 2. Hydrocarbon-rich stream 7 can, for instance, be used as fuel.Water-rich vapour stream 9 is withdrawn from the first distillationcolumn 2 and passed into the bottom of second distillation column 10.Top fraction 12 is passed through heat exchanger 13 and the resultingcooled stream 14 is the clean water stream, part of which (stream 15) isre-introduced into the top part of second distillation column 10. Thebottom fraction 11 of second distillation column 10 is led back to firstdistillation column 2 and enters this column below the vapour draw off9. Part of the bottom stream 16 of first distillation column 2 isrecovered as concentrated brine product, while the remaining part ispassed through reboiler 17 after which the resulting stream 18 isre-introduced into the bottom part of first distillation column 2.

Not shown in FIG. 1, but also possible is that part of condensed stream5 is led back into distillation column 2 and part is recovered as ahydrocarbon/water-containing bleed stream for e.g. fuel purposes. Inthat case phase separator 6 can be dispensed with.

What is claimed is:
 1. A process for treatment of a waste water feedstream originating from the production of styrene monomer and propyleneoxide and containing at least hydrocarbons and salts to yield a cleanwater product, a concentrated brine product and a hydrocarbon-richproduct, which process comprises the steps of: (a) feeding the wastewater feed stream into a first distillation column at a stage in therange of from 0.05n to 0.15n from the top, wherein n represents thetotal number of theoretical stages of the first distillation column andhas a value in the range of from 20 to 40; (b) drawing off a vaporstream at a stage in the range of from 0.55n to 0.75n from the top andfeeding said vapor stream into the bottom of a second distillationcolumn having m theoretical stages, with m having a value in the rangeof from 3 to 10; (c) drawing off the clean water product as the topfraction from the second distillation column and drawing off a bottomstream from the second distillation column, which is fed back into thefirst distillation column below the draw off of the vapor stream in step(b) at a stage in the range of from 0.60n to 0.85n from the top; (d)drawing off the concentrated brine product as the bottom fraction of thefirst distillation column; and (e) drawing off the hydrocarbon-richproduct as the top fraction of the first distillation column.
 2. Aprocess according to claim 1, wherein n has a value in the range of from25 to
 35. 3. A process according to claim 1, wherein the vapor stream isdrawn off in step (b) at a stage in the range of from 0.60n to 0.70n andthe bottom stream from the second distillation is fed back into thefirst distillation column in step (c) at a stage in the range of from0.65n to 0.75n from the top.
 4. A process according to claim 1, whereinm has a value in the range of from 3 to 7.